During the production of Iran's 217 metros, gear reduction boxes are a large irregular thin-walled cast steel box, and during production, due to the influence of factors such as casting flaws of porosity and blowholes of the gearbox, easy deformation during processing, device status and staff status etc, the gearbox production efficiency and yield rate cannot meet the production requirements. Thus it is necessary to have a reasonable set of processing scheme to overcome various unfavorable factors in the production, and to improve the processing quality and efficiency of the gear reduction box.